1. Field of the Invention
The present invention relates to a separator for a proton exchange fuel cell using solid polymer membrane as electrolyte and to a method of manufacturing the separator. More particularly, the present invention relates to a method of manufacturing a compact, light weight, separator for a proton exchange fuel cell with improved reliability and at low cost.
2. Description of the Related Art
A fuel cell is a device that converts chemical energy into electric energy by bringing a fuel such as hydrogen and an oxidizing agent such as air to electrochemically react with each other.
Various types of fuel cells which differ according to type of electrolyte used as known; for instance, phosphoric acid type, fused carbonate type, solid oxide type and proton exchange type. Of these fuel cells, a proton exchange fuel cell is a fuel cell utilizing the fact that when a polymer resin membrane containing a proton exchange radical is saturated with water it acts as proton conductive electrolyte. The proton exchange fuel cell acts in a relatively low temperature range with excellent power generating efficiency and has attracted attention in recent years.
FIG. 6 is a diagram showing the structure of a unit cell that is a base unit of a conventional proton exchange fuel cell.
As shown in FIG. 6, a unit cell 1 is composed of an ion conductive solid polymer membrane 2, an anode electrode 3 and a cathode electrode 4 arranged with solid polymer membrane 2 interposed between them. Further, at the outsides of these electrodes 3, 4, there are arranged an anode electrode side separator 5 and a cathode electrode side separator 6, each of which is gas impermeable and has a gas supply groove for supplying reaction gas to one of electrodes 3, 4.
As ion conductive solid polymer membrane 2, for instance, perfluorocarbon-sulfonic acid (Nafion-R: Du Pont, U.S.A.), which is a proton exchange membrane, is known. Solid polymer membrane 2 contains a hydrogen ion exchange radical, and functions as an ion conductive electrolyte when saturated in water. Solid polymer membrane 2 also functions to separate a fuel 7 supplied on the anode electrode 3 side of the solid polymer membrane 2 from an oxidizing agent 8 supplied on the cathode electrode 4 side of the solid polymer membrane 2.
Anode electrode 3 arranged at one side of solid polymer membrane 2 is formed of a catalytic layer 3a and a porous carbon flat plate 3b. Further, cathode electrode 4 arranged opposing anode electrode 3 is formed of a catalytic layer 4a and a porous carbon flat plate 4b. 
Separator 5 at the anode electrode side is composed of a separator substrate 9 and fuel supply grooves 10a, 10b arranged at both sides of separator substrate 9 for supplying fuel.
On the other hand, separator 6 arranged at the cathode electrode side is composed of a separator substrate 11, an oxidizing agent supply groove 12 for supplying an oxidizing agent arranged on one surface of separator substrate 11 at the surface side contacting cathode electrode 4, and a fuel supply groove 10 for supplying fuel arranged on another surface of separator substrate 11.
The principle of unit cell 1 will be described below.
When fuel 7 is supplied to anode electrode 3 and oxidizing agent 8 is supplied to cathode electrode 4, the electromotive force is generated by the electrochemical reaction between a pair of electrodes 3, 4 of unit cell 1. Normally, hydrogen is used as fuel 7 and air is used as oxidizing agent 8.
When hydrogen is supplied as fuel to anode electrode 3, hydrogen is ionized into hydrogen ion and electron in anode catalytic layer 3a (Anode reaction). The hydrogen ion moves to cathode electrode 4 through solid polymer membrane 2, and the electron moves to cathode electrode 4 through an external circuit. On the other hand, the oxygen contained in the air is supplied to cathode electrode 4 as oxidizing agent 8 causes the cathode reaction by the hydrogen ion and the electron in catalytic layer 4a to generate water. At this time, the electrons pass through the external circuit and become a current and is able to feed electric power. In other words, in anode electrode 3 and cathode electrode 4, reactions shown below will progress. Further, the generated water is discharged together with not-reacted gas to the outside of unit cell 1.
Anode Reaction: H2xe2x86x922H++2exe2x88x92
Cathode Reaction: 2H++1/202+2exe2x88x92xe2x86x92H2O
In such unit cell 1, if water content in solid polymer membrane 2 becomes less, ion resistance becomes high, and mixing of fuel 7 and oxidizing agent 8 (crossover) takes place, and unit cell 1 is not able to generate the electric power. So, it is desirable to keep solid polymer membrane 2 in the state saturated with water.
Further, when the hydrogen ion ionized in anode electrode 3 upon power generation moves to cathode electrode 4 through solid polymer membrane 2, water also moves jointly. So, at the anode electrode 3 side, solid polymer membrane 2 tends to become dry. Further, if moisture contained in supplied fuel 7 or supplied air is less, solid polymer membrane 2 tends to become dry at around respective inlet ports of reaction gases. For this reason, pre-humidified fuel 7 and pre-humidified oxidizing agent 8 are generally supplied to unit cell 1.
By the way, electromotive force of unit cell 1 is low as below 1 volt, and a cell stack is generally formed by laminating several tens to several hundreds of unit cells 1 via separators 5, 6 arranged at the upper and lower sides of unit cells 1. Cooling plates are inserted into respective unit cells 1 in order to control the temperature rise of the cell stack resulting from the power generation.
Separators 5, 6 used in a proton exchange fuel cell are required to be impermeable to reaction gas and cooling water so as to separate each of unit cells 1. On the other hand, separators 5, 6 are also required to be electrically conductive in order to laminate unit cells 1 to provide a cell stack and to function as the fuel cell. Normally, a proton exchange fuel cell is operated at relatively low temperature of 70xcx9c90xc2x0 C. Separators 5, 6 inside the proton exchange fuel cell are under the severe environment where they are exposed to the air containing water vapor whose vapor pressure is close to a saturate vapor pressure at the temperature of 70xcx9c90xc2x0 C., and at the same time, potential difference is generated between separators 5, 6 pursuant to the electrochemical reaction. So, it is necessary to select a corrosion proof material for the separators 5,6. As corrosion proof material, stainless steel, etc. are generally used. When stainless steel, etc. are applied to separators 5, 6, the surface thereof is oxidized and a passive state membrane is formed on the surface thereof. As a result, the resistance loss of the fuel cell becomes large and power generating efficiency drops to a large extent.
In the U.S.A., during 1970""s, for the separators of the proton exchange fuel cell developed for the space shuttle, niobium which is excellent corrosion proof noble metal, was used. However, noble metal materials have such defects that they are extremely expensive and heavy. So, as disclosed in U.S. Pat. No. 5,521,018, Ballard Power Systems Inc. of Canada uses carbon plates for separators so as to reduce the weight and cost of a cell stack.
FIG. 7 shows the construction of a cell stack of a conventional proton exchange fuel cell using carbon plates for separators.
As sown in FIG. 7, a cell stack 13 is composed of, in an outer frame 14, a cell portion 15 which generates electric power by reacting gas, and a humidifying portion 16 for humidifying reaction gas. In cell portion 15, a plurality of unit cells 1 are arranged in outer frame 14.
FIG. 8 is a schematic diagram showing the structure of conventional unit cell 1 in cell portion 15 shown in FIG. 7.
As shown in FIG. 8, in unit cell 1 arranged in cell portion 15 is in the structure as described below. Anode electrode 3 and cathode electrode 4 are arranged with ion conductive solid polymer membrane 2 interposed between them. A cooling separator 17 is provided at the outside of anode electrode 3, and anode electrode side separator 5 is provided at the further outside of cooling separator 17. Further, at the outside of cathode electrode 4, cathode electrode side separator 6 is provided.
Cooling separator 17 is provided to prevent the heating of the cell portion 15 by absorbing the reaction heat generated from the reaction by cooling water.
FIG. 9 is a plan view showing the construction of conventional separator 6 provided at the cathode electrode side.
As shown in FIG. 9, separator 6 is composed of a nearly square shaped separator substrate 11 made of a carbon plate with an air induction port 18 for inducing air and a fuel gas induction port 19 for inducing fuel gas provided at one corner of separator substrate 11. At the side opposing these induction ports 18, 19, an air discharging port 20 for discharging air and a fuel gas discharging port 21 for discharging fuel gas are provided. Further, at other corners of separator substrate 11, a cooling water induction port 22 and a cooling water discharging port 23 are provided. On separator substrate 11, a serpentine shaped air groove 24 is formed for inducing air to the reaction surface. Air groove 24 connects air induction port 18 and air discharging port 20. Though not shown in FIG. 9, at the under side of separator substrate 11, the fuel supplying groove is formed, which connects fuel gas induction port 19 and fuel gas discharging port 21.
Air groove 24 is formed on separator substrate 11 made of relatively soft carbon plate by applying the press working at one surface thereof. Further, the fuel supplying groove is formed on separator substrate 11 by applying the press working at another surface thereof.
Further, although not illustrated here, separator 5 at the anode electrode side and cooling separator 17 have almost the same structure as that of separator 6 at the cathode electrode side.
The structure of humidifying portion 16 shown in FIG. 7 is almost the same as that of cell portion 15. In cell portion 15, reaction gasses mutually contact via solid polymer membrane 2. However, in humidifying portion 16, air or fuel gas, that is reaction gas, is humidified by contacting cooling water via a steam transmission membrane.
However, even in the case of separators 5, 6 as described above, there is still a restriction for making the thickness of separators 5, 6 thin.
As reasons for this, it is pointed out that first, in the case of a proton exchange fuel cell using a carbon plate for separators 5, 6, a certain thickness is needed for the carbon plate in order to maintain the strength as separators 5, 6. Secondly, a carbon plate is intrinsically porous material and it is necessary to prevent transmission of gas and water between the separators, and therefore, it is restricted to make the thickness of separators 5, 6 thin. In the U.S. Pat. No. 5,521,018, the thickness of separators is 1.6 mm and a certain thickness is demanded.
To make cell stack 13 compact, it is most important to make the thickness of unit cell 1 thin. However, when a carbon plate is applied to a separator, there is such a problem that, it is restricted to make the separators thin and it is difficult to make cell stack in a compact size.
Furthermore, as the carbon material itself is expensive, there is such a problem that it is difficult to provide a cell stack 13 at low cost.
Furthermore, as the carbon plate has lower thermal conductivity than that of such metals as aluminum, copper, etc., it is needed to cool down unit cells 1 by inserting cooling plates 17 to flow cooling water between respective unit cells 1. Accordingly, there is such a problem that a cell stack becomes a larger size, and air-cooling is difficult to adopt in the proton exchange fuel cell.
On the other hand, in the case wherein metal is used as separator substrate instead of carbon plate, there is such a problem that the corrosion is generated when using the separators in the saturated steam atmosphere or by the potential difference generated specifically to the fuel cell and thereby the cell performance drops.
Further, in the case of a metallic separator, it is considered that the separator is formed principally according to the pressing or machining. The separator can be formed at low cost by the pressing. But in the case of a separator containing material composed of a light metal as a main component, such as aluminum, magnesium, etc., the deformation of the separator by forming pressure, the wrinkles and break by the structural flow tend to be caused, and as a result, it was difficult to process the separator precisely.
Accordingly one object of this invention is to provide a separator of a proton exchange fuel cell in compact size and light weight at low cost.
Another object of this invention is to provide a manufacturing method of a separator of a proton exchange fuel cell in compact size and light weight at low cost.
These and other objects of this invention can be achieved by providing a method of manufacturing the separator of the proton exchange fuel cell. In a cell stack of a proton exchange fuel cell, the cell stack composed by laminating a plurality of unit cells and a plurality of separators, each of the unit cells composed of an anode electrode, a cathode electrode and a solid polymer electrolytic membrane arranged between the anode and cathode electrodes, each of the separators arranged between the unit cells, respectively, the method of manufacturing the separator of the proton exchange fuel cell, includes the steps of, preparing step for preparing raw material for the separator, the raw material composed of alloy material including light metal as a main component, and forming step for forming the separator from the raw material by die casting.
According to one aspect of this invention, there is provided a method of manufacturing the separator of the proton exchange fuel cell, wherein in the alloy material, the light metal includes Al or Mg.
As features of the die casting, a short filling time, high speed filling, high pressure filling and fast cooling speed of molten metal are enumerated. So, according to this invention, by using the die casting, a separator can be formed with the highly productivity, and excellent dimensional accuracy, requiring less post process. Further, a separator which is provided with smooth cast surface, is mechanically strong and can be formed thin is possible according to this invention.
So, according to this invention when separators are manufactured according to the die casting using an alloy material composed of Al or Mg, that is light metal, as a main component, it becomes possible make a cell stack light and compact. Further it becomes possible to manufacture a large quantity of separators in a short period of time, and thereby to reduce manufacturing cost per separator. Further, when an alloy material composed of Mg as a main component is used, it is possible to make the weight of separator and cell stack more light.
According to one aspect of this invention, there is provided a method of manufacturing the separator of the proton exchange fuel cell as described above, wherein in the alloy material includes one of alloy materials described below:
an alloy material composed of the following components (in weight %): Cu: 0.3xcx9c0.4%, Si: 12.0xcx9c12.2%, Mg: 0.08xcx9c0.11%, Mn: 0.23xcx9c0.24%, Fe: 0.64xcx9c0.74%, Zn: 0.09xcx9c0.10%, and Al: the remainder;
an alloy material composed of the following components (in weight %): Cu: 2.8xcx9c3.0%, Si: 8.4xcx9c8.6%, Mg: 0.08xcx9c0.10%, Mn: 0.32xcx9c0.37%, Fe: 0.70xcx9c0.71%, Zn: 0.13xcx9c0.14%, and Al: the remainder;
an material composed of the following components (in weight %): Cu: 1.9xcx9c2.5%, Si: 9.4xcx9c10.4%, Mg: 0.21xcx9c0.25%, Mn: 0.16xcx9c0.38%, Fe: 0.72xcx9c0.93%, Zn: 0.42xcx9c0.93%, Al: the remainder;
an material composed of the following components (in weight %): Cu: 0.01%, Si: 0.1%, Mn: 0.80%, Fe: 0.09%, Ni: 0.01%, Ti: 0.01% and Al: the remainder;
an alloy material composed of the following components (in weight %): Mn: 1.4xcx9c4.3%, Fe: 0.5xcx9c1.0%, and Al: the remainder;
an alloy material composed of the following components (in weight %): Cu: 0.05%, Co: 1.0% and Al: the remainder; and
an alloy material composed of the following components (in weight %): Mn: 2.0%, Zn: 3.0%, Fe: 1.0%, Mg 0.5% and Al: the remainder.
When alloy materials composed of Al as a main component, with the composition stated as described above, are used, it is possible to manufacture highly reliable separators with good fluidity during the die casting and less casting defects. Further, it is possible to make highly strong separators which are thermally as well as electrically conductive.
According to another aspect of this invention, there is provided a method of manufacturing the separator of the proton exchange fuel cell as described above, wherein in the alloy material includes one of alloy materials described below:
an alloy material composing of the following components (in weight %): Al: 5.3xcx9c6.1%, Zn: 2.5xcx9c3.5%, Mn: 0.15xcx9c0.6%, Si: below 0.3%, Cu: below 0.1%, Ni: below 0.01%, and Mg: the remainder;
an alloy material comprising the following components (in weight %): Al: 8.1xcx9c9.3%, Zn: 0.4xcx9c1.0%, Mn: 0.13xcx9c0.5%, Si: below 0.3%, Cu: below 0.1%, Ni: below 0.01%, and Mg: the remainder;
an material comprising the following components (in weight %): Al: 8.3xcx9c9.7%, Zn: 1.6xcx9c2.4%, Mn: 0.1xcx9c0.5%, Si: below 0.3%, Cu: below 0.1%, Ni: below 0.01%, and Mg: the remainder;
an alloy material comprising the following components (in weight %): Al: 9.3xcx9c10.7%, Zn: 0.3%, Mn: 0.1xcx9c0.5%, Si: below 0.3%, Cu: below 0.1%, Ni: below 0.01%, and Mg: the remainder;
an alloy material comprising the following components (in weight %): Zn: 3.6xcx9c5.5%, Zr: 0.5xcx9c1.0%, Cu: below 0.1%, Ni: below 0.1%, and Mg: the remainder;
an alloy material comprising the following components (in weight %): Zn: 5.5xcx9c6.5%, Zr: 0.6xcx9c1.0%, Cu: below 0.1%, Ni: below 0.01%, and Mg: the remainder;
an alloy material comprising the following components (in weight %): Zn: 2.0xcx9c3.1%, Re: 2.5xcx9c4.0%, Zr: 0.5xcx9c1.0%,. Cu: below 0.1%, Ni: below 0.01%, and Mg: the remainder;
an alloy material comprising the following components (in weight %): Th: 2.5xcx9c4.0%, Zr: 0.5xcx9c1.0%, and Mg: the remainder;
an alloy material comprising the following components (in weight %): Th: 2.5xcx9c4.0%, Zn: 1.7xcx9c2.5%, Zr: 0.5xcx9c1.0%, and Mg: the remainder;
an alloy material comprising the following components (in weight %): Th: 1.4xcx9c2.2%, Zn: 5.2xcx9c6.2%, Zr: 0.5xcx9c1.0%, and Mg: the remainder;
an alloy material comprising the following components (in weight %): Zn: 2.0xcx9c3.5%, Zr: below 0.5%, and Mg: the remainder; and
an alloy material comprising the following components (in weight %): Zn: 3.6xcx9c5.5%, Zr: 0.5xcx9c1.0%, and Mg: the remainder.
When alloy materials composed of Mg as a main component, with the composition stated as described above, are used, it is possible to manufacture highly reliable separators with good fluidity during the die casting and less casting defects. Further, it is possible to make highly strong separators which are thermally as well as electrically conductive. Further, when an alloy material composed of Mg as a main component is used, it is possible to make the weight of separator and cell stack more light.
According to one aspect of this invention, there is provided a method of manufacturing the separator of the proton exchange fuel cell, wherein the alloy material has thermal conductivity of 100xcx9c230 W/mK.
According to the present invention, heat generated during the operation can be efficiently cooled down. Further, because the separator is manufactured of Al alloy material of a light specific gravity, the cell stack can be made light and compact.
According to another aspect of this invention, there is provided a method of manufacturing the separator of the proton exchange fuel cell, wherein: the alloy material has electric conductivity of 30xcx9c60%.
According to the present invention, it is possible to reduce the electric contact resistance of separators, which is the important cell performance. As a result, it is possible to eliminate the voltage drop caused by the resistance of a unit cell composed of an anode electrode and a cathode electrode arranged on both sides of a solid polymer electrolytic membrane.
According to one aspect of this invention, there is provided a method of manufacturing the separator of the proton exchange fuel cell, wherein in the forming step, the separator is formed from the raw material by the die casting such that a thickness of the separator is 0.4 mm or more.
According to the present invention, as the deformation of the separator caused during the manufacturing by the die casting can be prevented, and the generation of casting defects can be minimized. As a result, a highly reliable cell stack can be made light and compact.
According to one aspect of this invention, there is provided a method of manufacturing the separator of the proton exchange fuel cell, wherein in the forming step, the die casting includes one kind or a composite process of two or more kinds of vacuum die casting, oxygen atmospheric die casting, atmospheric flow die casting, rheo-casting, acurad process, low speed filling die casting, high pressure casting, balance type flow die casting, car process, and parashot process.
According to this invention, it is possible to manufacture thin and complicated shaped separators in excellent accuracy in a short time at a low cost.
According to one aspect of this invention, there is provided a method of manufacturing the separator of the proton exchange fuel cell, further including the step of trimming step for trimming the separator formed by the die casting and treated by the surface treating step according to one of press machining, electric discharge machining, laser beam machining and photo-etching.
According to the present invention, it becomes possible to make additionally the machining of an opening and the forming of a thin portion that are difficult in the die casting. As a result, a thin and complicated shaped separator can be manufactured easily at a low cost.
According to one aspect of this invention, there is provided a method of manufacturing the separator of the proton exchange fuel cell, further including the step of surface treating step for treating a surface of the separator formed by the die casting according to one kind or a composite process of two or more kinds of buffing, barrel finishing, blasting, electro-polishing, and chemical treatment, such that surface roughness of the separator is 0.07 xcexcm or more.
Furthermore, the surface of the separator after trimmed by the trimming step is treated again by the surface treating step as describe above.
According to the present invention, it is possible to completely remove burr, cohesion, rough casting surface produced in the die casting. As a result, it is possible to minimize the detects in the coating film during the corrosion resistant coating that is applied in the post processing, and thereby to improve the adhesion of the separator substrate with the coating film.
According to another aspect of this invention, there is provided a method of manufacturing the separator of the proton exchange fuel cell, further including the step of coating step for forming corrosion resistant coating film on the surface of the separator after the surface of the separator is made rough by the surface treating step according to one kind or a composite process of two or more kinds of physical evaporation process, plating process and spray process.
According to the present invention, it is possible to prevent the formation of passive state film of oxides produced in the saturated steam or in the potential corrosion environment specific to the fuel cell, and thereby to obtain a stabilized fuel cell output for a long time.
According to one aspect of this invention, there is provided a method of manufacturing the separator of the proton exchange fuel cell, further including the step of inspection step for conducting non-destructive inspection on the surface of the separator, after the die casting or after the corrosion resistant coating film is formed, according to one of infrared thermography, eddy current testing, magnetic particle testing, penetration inspection, ultrasonic testing, electromagnetic induction inspection, acoustic emission testing, macroscopic inspection, radiograph inspection, laser holography inspection and acoustic inspection.
According to the present invention, casting defects caused during the die casting and defects in the coating film produced during the corrosion resistant coating can be detected quickly by performing the non-destructive inspection as described above. As a result, it is possible to reflect the result of the inspection on the improvement of manufacturing conditions and thereby to manufacture highly reliable separators.
According to one aspect of this invention, there is provided a separator of the proton exchange fuel cell, wherein the separator is manufactured by using the one of the methods of manufacturing the separator of the proton exchange fuel cell as described above.